This invention pertains to power supplies for X-ray tubes and more particularly concerns circuitry for stabilizing X-ray tube current.
X-ray tubes are commonly of Collidge type which is a diode having a thermonic cathode which emits electrons, at least some of which strick an anode called a plate. The plate emits X-ray radiation, the intensity of which being dependant upon the voltage imposed across cathode and plate (tube voltage) and the current between cathode and plate (tube current). The thermonic cathode is heated by a filament. A factor affecting tube current is filament temperture which corresponds to filament current. The higher the filament current, the higher will be the tube current. A number of circuits are known which cause the filament current to respond inversely to tube current so as to maintain a constant tube current regardless of moderate fluctuations in line voltage.
An example of a known circuit is illustrated in FIG. 1 which is labeled "Prior Art". Connected across the plate 10a and cathode 10b of X-ray tube 10 is the high voltage output of transformer 12. The input of transformer 12 is driven by alternating line voltage. The tube 10 is a half wave rectifier as it only passes tube current during alternate half cycles. The voltage drop across a series resistor 14 is proportional to tube current. This waveform is shown in FIG. 2 and identified as tube current signal.
Tube filament 10c is heated by filament current supplied by a low voltage filament transformer 16 driven by line voltage through control circuitry. Uncontrolled filament current is a full wave sinusoid.
A sample of the tube current signal is connected across a trigger diode 18 or silicon unilateral switch or the like. When the signal exceeds a threshold the diode 18 triggers a controlled rectifier 20 on by passing a trigger signal through an isolation transformer 22. When the controlled rectifier 20 is on, it shunts a resistor 24 across the input of filament transformer 16. Resistor 24 and resistor 26 forms a voltage divider with transformer 16, resulting in lower input voltage to transformer 16 and corresponding lower filament current. The controlled rectifier 20 remains closed until the end of the particular half cycle.
This scheme controls tube current by monitoring tube current. When the tube current exceeds a predetermined value the filament current is lowered for the remainder of the half cycle so as to maintain constant tube current even with moderate variations of line voltage.
The waveform of the controlled filament current is shown in FIG. 2. It is seen that since the controlled rectifier only regulated during the half cycle when the tube is conducting the filament current is unsymmetrical. This has the undesirable effect of providing a direct current bias which may saturate the filament transformer.